There is an increasing need for inexpensive sensitive solid state gas sensors for such applications as pollution control, toxic gas monitoring, homeland security, “lab-on-a-chip,” etc. It is known that certain materials, such as metal oxides, exhibit sensitivity to various gases due to phenomena at the material surface. Recently, solid state gas sensors have been proposed and fabricated that employ the use of planar thin films of these sensitive materials. Eranna et al., Oxide materials for development of integrated gas sensors—A comprehensive review, Critical Reviews in Solid State and Materials Sciences 29 (3-4): 111-188 (2004), describes use of solid state metal oxides as sensors for various gases.
One type of microelectronic sensor device is known as a CHEMFET, as shown in prior art FIGS. 1 and 2, wherein a pH monitor is depicted in FIG. 1, generally at 10, Fraden, Handbook of Modern Sensors, 2d Ed., AIP Press, p. 499 (1996). pH monitor 10 includes a CHEMFET sensor device 12, which is encapsulated in a material 14, and works with a reference electrode 16 to determine the pH of a gas. The pH sensor uses silicon dioxide 18 as a gate oxide, which is covered by another gate insulator layer 20, in this case, Si3N4. A schematic diagram of pH monitor 10 is shown in FIG. 2, in a source-follower mode. These devices are well known and have been used for monitoring quantities, such as pH. The structure of a CHEMFET is similar to that of an MOS transistor in which the gate terminal is replaced by a chemically sensitive film that is left open to the environment. Exposure to gases, liquids, impurities, etc. may modify the surface of the film, changing the surface charge state. The surface field effect modulates charge carrier concentration (conductivity) in the channel. The effect is equivalent to application of a bias to a MOS gate. The current measured between the source and drain terminal is related to the changes in concentration of the substance being sensed. Advantages of this device structure are low power consumption and high sensitivity, which are the results of the FET configuration.
The sensitivity of many known materials, such as In2O3, SnO2 and ZnO, to various gases has been attributed to surface phenomena. Therefore, nanostructured materials, because of their inherent high surface area, should be ideal for sensing applications. However, a problem has been how to fabricate a useful, i.e., electrically measurable and MOS integratable, device using nanostructured materials.
Martins et al., Zinc oxide as an ozone sensor, J. Appl. Phys. 96(3), 1398 (2004), describes the use of a UV bombarded ZnO film-on-glass as a sensor.
Gordillo et al., Effect of gas chemisorption on the electrical conductivity of ZnO thin films, Advances in Mat. Sci. and Tech. 1(1), 1 (1996), describes use of an annealed ZnO thin film as a detector for CO2, O2, H2 and CH4.
Zhang et al., Low-temperature growth and Raman scattering study of vertically aligned ZnO nanowires on Si substrates, APL 83, 4632 (2003), describes formation of ZnO nanowires on a gold-catalyzed silicon substrate.